What is fretboard radius?
Fretboard radius is simply the curve of the fretboard from side to side. This curve is to make the fretboard more comfortable for the fretting hand. The radius is described as a measurement such as 7.25", 9.5", 12" or 16". These measurements tell you that the fretboard is a segment of a circle or cylinder, which has a radius, or size of 7.25", 9.5", 12" or 16". The larger the radius, the larger the circle, and the flatter the fretboard will be.
In simple terms
Quite simply, the smaller radius (which is more curved) is more comfortable for playing (particularly barre) chords, while a larger, flatter radius is better for low action, single-note playing and bending. The other important characteristic of radius is that the flatter the radius, the lower the action can be. This is because when you bend a string on a lower part of the fretboard such as your first or second string, you are bending it towards the middle of the fretboard, which is higher than the edges, so the notes will tend to "fret out" - buzz against the higher parts of the fret, killing sustain.
Another important thing to note is that staggered polepiece pickups (like vintage style single-coils) were originally designed for vintage Fender guitars, which had a small 7.25" radius. These sometimes don't work well with flatter radii, making the middle strings jump out much louder, particularly the G string (which was also designed for wound strings). There is a workaround for this problem, which is to keep the pickups set lower (further away from the strings), where the string-to-string balance will even out. The downside to this workaround, is that you lose volume with the pickups set lower. You should also be aware that there are "modern vintage" pickups, which also have staggered polepieces.
Which fretboard radius is best?
Which radius is "best" tends to be a personal thing, you should be asking "which is best for me?". Generally, most players are going to prefer something between a 9.5" and a 12". Many players who play more "lead-based" styles, or those needing lower action and more bending will probably be happier with larger radii such as 16" or even 20". Ultimately this is down to personal preference, which is largely based on the instruments we learned to play on. The best thing you can do is play as many different guitars as possible (like we need an excuse, right?), making note of which feel more comfortable to you. Bear in mind that other factors such as neck thickness, fretboard thickness, scale length and even finish do affect the feel of a neck, but the more time you spend playing different instruments, the easier it will be to identify each factor.
The fingerboard, or playing surface of the guitar neck, is usually curved or radiused across its width. The purpose of this radius is to accommodate the natural ergonomic shape of your fingers when they are in playing position. Comfort is certainly a major factor in selecting a fingerboard radius but it is not the only factor. Musical Style... the fingerboard radius must also allow you to play the style of music you prefer.
Compound Radius Fingerboard
(A Warmoth innovation)
This is a concept that we introduced nearly two decades ago to improve both comfort and playability. It has proven to be a very popular feature that we have incorporated into the majority of our necks.
Everyone knows how comfortable vintage Fender necks are for rhythm work and chording, but without uncomfortably high action, string bending is not an option the strings "fret-out".
To achieve low action and no buzz string bending, many necks resort to a 16" fingerboard radius. This certainly works, but the comfort factor is lost.
What we developed was the theory and technology to make the fretboard conical. This retains a tighter radius in the area commonly used for rhythm and chording, while flattening the area used for bending and lead playing. We selected a 10" radius at the nut for both comfort and compatibility with the popular Floyd Rose locking nut. A 16" radius at the heel has proven to afford 2-1/2 step bends with action below a 16th of an inch!
In usage, the changing radius is not really noticed. It is simply easier to play on and more comfortable. Compared to a conventional single radius neck, the compound radius is far more difficult and time consuming to produce. So, why do we do it? This is a design that will make a genuine difference in your playing. An improvement in your playing speed... Its fast! String bending has never been easier, and comfortable. And of course your playing will be better if your hand is more relaxed.
One final note about the compound radius
There is no practical difference in accomplishing a fret level, nut cut, set-up or any other adjustment to a compound radius neck. They are as easy to work with as any other neck.
Custom Straight Radius
Straight radius necks have been the industry standard since the inception of guitars and are still used by the majority of manufacturers; primarily because it is much easier to produce than the much more comfortable compound radius necks. For those of who prefer straight radius, we have a custom machine which is adjustable to any straight radius between 9" and 16", in half inch increments. We can offer this custom work on our Warmoth "Pro" and Vintage Modern construction necks, in either 25 1/2" or 24 3/4" scale lengths.
(Article taken from Warmoth.com)